Periodic polarization reversal elements using a ferroelectric crystal substrate are employed as wavelength conversion elements which are used for obtaining laser light of a desired wavelength and the like. In a periodic polarization reversal element, a periodic polarization reversal structure whereof the polarization direction reverses periodically is formed on a ferroelectric crystal substrate. For example, a periodic polarization reversal element can output laser light of a wavelength which is a second harmonic by performing quasi phase matching on the inputted laser light. Periodic polarization reversal elements are thus used as quasi phase matching (QPM) type wavelength conversion elements.
To form a periodic polarization reversal structure, methods are employed such as reversing the spontaneous polarization direction by applying voltage between electrodes arranged on the +Z surface and −Z surface perpendicular to the polarization direction of the ferroelectric crystal substrate (for example, see Patent Literature 1). The shape and area of the periodic polarization reversal structure to be formed are determined by the shape of the electrodes arranged on the ±Z surfaces. Generally, the electrodes arranged on the ±Z surfaces are formed by forming a metal into film.
The metal electrodes arranged on the +Z surface comprise stripe electrode sections formed from a continuous metal film and arranged, for example, at a fixed pitch, and an equipotential electrode section which is connected to the stripe electrode sections in order to keep the stripe electrode sections at a uniform potential. Specifically, comb-shaped electrodes and ladder-shaped electrodes have been used. The electrode on the −Z surface which opposes the electrodes formed on the +Z surface across the ferroelectric crystal substrate is formed as a metal film solid electrode (planar electrode).
For example, a predetermined voltage is applied between a ladder-shaped electrode (top surface electrode) arranged on the +Z surface of the ferroelectric crystal substrate and a solid electrode (bottom surface electrode) arranged uniformly over the −Z surface of the ferroelectric crystal substrate. At this time, due to the electric field generated in the ferroelectric crystal substrate, polarization reversal occurs directly below the stripe electrode sections (cross-piece parts) on the +Z surface, and a periodic polarization reversal structure is formed inside the ferroelectric crystal substrate.